1. Field of the Invention
The present invention relates to a variable-power optical system or a projection optical system, and the present invention is suitably applied to, for example, a projection optical system of an image projection apparatus which enlarges an original image and projects the enlarged original image on a screen (projection surface).
2. Related Background Art
Heretofore, a variety of liquid crystal projectors (image projection apparatuses), each of which uses an image display element such as a liquid crystal display element and projects an image (original image) displayed on the image display element on a screen surface (projection surface), have been proposed.
In particular, as an apparatus capable of projecting an image of a personal computer and the like on a large screen, such a liquid crystal projector has been widely utilized in a conference, a presentation, and the like. Moreover, recently, a downsized liquid crystal projector for a home theater system has been widespread, and along with this, a size of the liquid crystal display element tends to be also downsized.
The liquid crystal projector as described above is roughly classified into two types. One of the types is a transmission liquid crystal projector which synthesizes colors of a modulated image transmitted through the liquid crystal display element and projects the color-synthesized image. The other type is a reflection liquid crystal projector which has a mirror installed on a back side of the liquid crystal display element, reflects a modulated image on the mirror, and projects the reflected modulated image.
In a liquid crystal projector of a three-plate system, which uses three liquid crystal display elements, a space in which a dichroic prism, a polarizing plate, and the like which synthesize color light modulated by the liquid crystal display elements are to be arranged must be secured between the liquid crystal display elements and a projection lens, and it becomes necessary to ensure a back focus with a certain length for the projection lens.
In such a liquid crystal projector, a negative lead type zoom lens in which a lens unit having negative refractive power is arranged in the front has been widely utilized. The negative lead type zoom lens has features that widening of a view angle thereof is relatively easy, it is possible to well maintain optical performance in a distance of close-up shooting, and so on. On the contrary, a movement of the lens unit moving at zooming is increased, and aberration variations are also increased along therewith. Moreover, the negative lead type zoom lens has drawbacks that it is difficult to impart a higher zoom ratio thereto, it is difficult to widen the angle while restricting various aberrations, and so on.
Moreover, what is required for a projection optical system for use in a color liquid crystal projector is as follows:                The projection optical system should be a so-called telecentric optical system in which a pupil on the liquid crystal display elements (reduction) side is present at an infinite distance for the purpose of minimizing an influence of a dependency on an angle of a color synthesis film provided on the dichroic prism and ensuring good pupil alignment with an illumination system;        Respective images on pixels of three colors must be superposed together on an entire screen so as to prevent a situation where a resolution and quality of a picture (image) on liquid crystal display elements of the respective colors are impaired, for example, letters and the like on a personal computer display look double when the picture (image) is synthesized and projected on the screen. To that end, a color misregistration (chromatic aberration of magnification) occurring on a projection lens should be corrected well in the visible light range;        For the projected image, a distortion should be sufficiently corrected lest the projected image should be distorted at an outline thereof and look unsightly (particularly, when abrupt changes of the distortion in a periphery, intermediate portion and the like thereof remain to be eliminated, image quality is undesirably lowered); and the like.        
Moreover, recently, for a small-size panel-equipped projector, downsizing and weight reduction thereof have been strongly required placing a premium on mobility while there have arisen needs such as higher brightness of the screen and higher definition of the image. Furthermore, specifications of higher brightness and a wider angle of view, which enable projection onto a bright and large screen in a narrow room, have also been required.
Heretofore, as such a projection lens for the liquid crystal projector, there has been known a six-unit zoom lens composed of six lens units as a whole, which have negative, positive, positive, negative, positive, and positive refractive powers sequentially from an enlargement side (front side), and by properly moving predetermined lens units thereamong, zooming is performed (for example, see Japanese Patent Application Laid-Open No. 2001-108900). This six-unit zoom lens sets first, fourth, and sixth lens units to be fixed, and moves second, third, and fifth lens units inside the lens system at the time of zooming from a wide angle end to a telescope end. Accordingly, the six-unit zoom lens is a telecentric zoom lens, in which an overall length of the lens units is kept constant even at the time of zooming, and the aberration variations at the time of chromatic aberration and zooming are restricted. However, in this six-unit zoom lens, a back focus length is insufficient for use in the reflection liquid crystal projector.
Besides the above, as a projection lens for the liquid crystal projector, there has been known a four-unit zoom lens composed of four lens units as a whole, which have negative, positive, negative, and positive refractive powers from an enlargement side, and by properly moving predetermined lens units thereamong, zooming is performed (for example, see Japanese Patent Application Laid-Open No. 2001-215410). This four-unit zoom lens sets first and fourth lens units to be fixed, and moves second and third lens units inside the lens system at the time of zooming from a wide angle end to a telescope end. Accordingly, the four-unit zoom lens is a telecentric zoom lens, in which an overall length of the lens units is kept constant, and a sufficiently long back focus and a relatively wide angle of view in consideration of the application to the reflection liquid crystal projector are provided. However, this four-unit zoom lens has a relatively large F-number for full aperture (hereinafter, the F-number represents it for full aperture), and does not obtain sufficient brightness.
Moreover, Japanese Patent Application Laid-Open No. H11-190821 (counterpart: U.S. Pat. No. 6,285,509) and Japanese Patent Application Laid-Open No. 2000-019400 (counterpart: U.S. Pat. No. 6,081,389) have been known to disclose a projection lens for a projector using transmission liquid crystal display elements.
At present, as a liquid crystal projector, a small-size and mobile one is desired. Moreover, a capability of short-distance projection, which becomes a great merit as one for the home theater system, that is, the widening of the angle of view of the liquid crystal projector is required. Furthermore, in recent years, for the purpose of higher brightness of a projected video image, reflection liquid crystal display elements having good aperture efficiency have been utilized frequently.
As a solution to a problem of how to downsize of the liquid crystal projector, first, the liquid crystal display elements must be small. However, when the same resolution is to be obtained by use of the small liquid crystal display elements, an aperture ratio of the liquid crystal display elements is lowered, and a size ratio of a region to be illuminated to a light emitting source (=size of the region to be illuminated/size of the light source) becomes gradually small. Accordingly, illumination efficiency is lowered in general, causing a problem that the brightness falls even if the downsizing of the liquid crystal projector can be realized.
In the case of the transmission liquid crystal display elements, as a dimension of the liquid crystal display elements becomes smaller, the aperture ratio is lowered due to a drive circuit thereof, and a light quantity is reduced. On the contrary, in the case of the reflection liquid crystal display elements, the drive circuit can be installed on a back side of a panel, and accordingly, the aperture ratio is increased, and the reduction in light quantity can be restricted. For this reason, recently, a bright zoom lens for the reflection liquid crystal projector, which has high aperture efficiency, is desired in order to obtain the higher brightness in an efficient manner. However, in the reflection liquid crystal projector, required is a projection lens having a longer back focus than that of the transmission liquid crystal projector in order to insert a polarization splitter optical system and the like between the projection lens and the liquid crystal display elements in addition to the color synthesis optical system such as the dichroic prism.
In addition to the above-described problem, a projection lens with brightness in which an F number at a wide angle end is approximately 3.0 or more is desired for the projection lens for the small-size projector in order to ensure a screen illuminance sufficient for an observation under an illumination of a fluorescent lamp.
Moreover, it is not preferable that a pupil position on the original image side vary by zooming and focusing. It is necessary that an aperture stop be fixed for the liquid crystal display elements or that variations of the pupil position be minimized by moving the plural lens units.
Specifically, for the small-size and lightweight reflection liquid crystal projector, a projection lens is desired, which has high brightness/resolution and small variations of the pupil position, and is capable of the short-distance projection with the sufficiently long back focus.